What shrub or similar can I plant that grow quickly to provide privacy?

I love my garden but am a bit clueless as I lived in flats all of my adult life until I moved here a few years ago.

We have flowers and some veg etc growing so I can do the basics but haven't ever dealt with shrubs or bigger things before! There is a large gap in the fence on one side (not my fence) that means the neighbours can look straight into our garden and it's getting a bit much with their children. I don't want to fall out with them but my attempts to ignore/politely tell them we are busy and want to be left alone do not work and as summer approaches I would like to be able to sit in my garden in peace. What can I plant along my side of the fence that will grow quite quickly to give a bit of privacy? It doesn't need to be a total block - but enough to stop the children next door from sticking their heads round/stepping through all the time!

The garden is a slight slope and that corner has a shed in and the remains of the fence so is quite shady. Any advice would be really appreciated.

Pyracantha seems like it could fit the bill! I have got children too but they are a bit older and stay over the other side of the garden so would be fine to just stay away from the thorns I think. The Berberis looks lovely too from my googling - love the reddish colours. The gap is about 5 foot so with those suggestions I might be able to do something that would both keep my garden to myself and look nice too. Thank you for everyone's help.